Pump energy wavelength stabilization

In a method of stabilizing pump energy, a gain medium is provided having an absorption coefficient that varies with wavelength. An absorption coefficient curve of the absorption coefficient or a range of wavelengths comprises peaks and valleys. Pump energy is generated at an operating wavelength within one of the valleys, at which the absorption coefficient is approximately at a minimum. The pump energy is transmitted through the gain medium. A portion of the pump energy is absorbed with the gain medium and laser light is emitted from the gain medium responsive to the absorbed pump energy. The non-absorbed pump energy (feedback pump energy) is fed back to the pump module. The operating wavelength of the pump energy is stabilized using the feedback pump energy.